Chapter 7 PowerPoint

Question 1

What is equilibrium?

Answer 1

Equilibrium: state achieved if the resultant force and net torque (moment) acting upon an object are equal to zero

Question 2

Equilibrium can be either ____ or ____?

Answer 2

Equilibrium can be either linear or rotational

Question 3

What is linear/translational equilibrium?

Answer 3

Linear or translational equilibrium: achieved when the net of the external forces acting upon a system is equal to zero
e.g., a tug-of-war

Question 4

What is rotational equilibrium?

Answer 4

Rotational equilibrium: achieved when the net of the external torques (moments) acting upon a system is equal to zero

Question 5

What is static equilibrium?

Answer 5

Defined as a system in linear and rotational equilibrium and possessing 0 linear or rotational velocity
The object is completely at rest

Question 6

What three conditions must occur for static equilibrium to be acquired?

Answer 6

3 conditions occur for a static equilibrium state to be acquired
∑▒〖𝐹𝑦=0〗
∑▒〖𝐹𝑥=0〗
∑▒〖𝑇=0〗
Sums of all Vertical, Horizontal, and Torques are zero

Question 7

What is dynamic equilibrium?

Answer 7

Don’t assume that because of 0 net force or Torque the object isn’t moving
Riding in an Elevator? Or Riding in a Car or Airplane?
Dynamic Equilibrium
A system in motion but it is experiencing no change in velocity or direction

Question 8

What equations must be satisfied for dynamic equilibrium?

Answer 8

Dynamic equilibrium requires satisfying all of the following equations:
ΣFx - mαx = 0 ΣFY - mαY = 0 ΣFZ - mαZ = 0
ΣTx - Ixα x = 0 ΣTY - IYαY = 0 ΣTZ - IZαZ = 0

Question 9

What is ΣF?

Answer 9

ΣF = the sum of the forces in a given direction

Question 10

What is ma?

Answer 10

ma = the product of the body’s mass and acceleration

Question 11

What is ΣT?

Answer 11

ΣT = the sum of the torques about the given axis

Question 12

What is Iα?

Answer 12

Iα = the product of the body’s rotational inertia and angular acceleration about the given axis

Question 13

What is stability?

Answer 13

Stability: the resistance of an object to having its equilibrium disturbed
The more stable the object, the more resistance it has against forces or torques that attempt to disrupt its equilibrium.

Question 14

What is balance?

Answer 14

Balance: the ability to control the current state of equilibrium, and it implies conscious effort and coordination

Question 15

Stability is more of a ____ term, whereas balance is a ____ reference?

Answer 15

Stability is more of a mechanical term, whereas balance is a neuromuscular reference

Question 16

What is stable equilibrium?

Answer 16

Defined as a state in which a large force or torque is required to disrupt an object’s current position.
Note the Pyramid example
A lower center of gravity requires a longer torque to be applied to push the object over.
Coaching tip for most sports.

Question 17

What is restorative force?

Answer 17

Restorative Force
Uses gravity to return the object to the original position
Gravity is a torque on the object.
Line of gravity still on the original side of the axis of rotation

Question 18

What is a disruptive force?

Answer 18

Disruptive force
Force applied caused the line of gravity to move on the opposite side of the axis of rotation.

Question 19

The more torque required to overcome the ____ ____, the more ____ the object is considered to be?

Answer 19

The more torque required to overcome the restorative torque, the more stable the object is considered to be.

Question 20

What is unstable equilibrium?

Answer 20

State in which very little force or torque is required to disrupt an objects current position
Even a small force applied creates enough torque for the pyramid to fall over.
COG is not lifted at all, it only falls.
Pencil example

Question 21

What is neutral equilibrium?

Answer 21

State in which an object has no tendency to fall in one direction or another.
Think of a Sphere
The only way to disrupt neutral equilibrium is by tilting the surface

Question 22

What is linear stability?

Answer 22

Linear Stability – the resistance of a body to having its linear equilibrium disrupted.

Question 23

Increased stability often accompanied by decreased ____?

Answer 23

Increased stability often accompanied by decreased mobility

Question 24

What is a major factor of an object that determines its stability?

Answer 24

Mass of Object – major factor
Greater force required to linearly accelerate a more massive object
Greater mass leads to increase friction force

Question 25

What is rotational stability?

Answer 25

Rotational Stability – the resistance of a body to having its rotational or angular equilibrium disrupted
Major factor – mass of object
> inertia(mass) of object the more Torque is require to disrupt equilibrium.
Major factor – location of center of gravity relative to Base of support.

Question 26

What is base of support?

Answer 26

The entire area formed by the objects points of contact with resistive surface providing counterforce.
Rotational stability is directly related to the location of the center of gravity.
COG in middle of base of support, then no force arm. Object stays in rotational equilibrium.

Question 27

Rotational stability is directly related to the location of the ____ ____ ____?

Answer 27

Rotational stability is directly related to the location of the center of gravity.

Question 28

How can rotational stability be increased?

Answer 28

Rotational Stability can be increased by:
Keeping COG in middle of Base of support
Lowering COG of object
Moving COG to one side of base of support as long as it’s the same side of the external force being applied.

Question 29

Describe the keys of the base of support?

Answer 29

Size and shape of BOS affects the ease with which the line of gravity falls outside the boundaries of the base.
A wide base in the direction of line of action of external force
The key is to put as much distance between the line of gravity and the potential axis of rotation as possible.

Question 30

Describe the relationship of rotational stability and mass?

Answer 30

Larger mass translates into a larger rotational inertia
Greater torque required to provide angular acceleration
Sumo wrestler will be stable both linearly and Rotationally as long as he maintains a wide stance and low center of gravity
Same is true for other sports

Question 31

How is stability relative in the medical world?

Answer 31

PT/OT
Work with patients in need of restoring Balance, controlling equilibrium after injury or from a congenital condition.
Proprioceptors – body’s built in sensory system communicating with the brain
Reporting information on Joint position, velocity, muscle tension, rate of change in length.
Tons of equipment for Balance training